The present invention relates generally to oils, and more specifically, to a method for evaluating a hydrocarbon oil during use to determine its remaining useful life.
It is common to lubricate and cool the components of equipment by wetting them with a hydrocarbon oil lubricant. Such a lubricant experiences various environmental stresses as it carries out such functions that cause its basestock to undergo thermal-oxidative degradation. For this reason, various antioxidants are added to the lubricant to protect its lubricating characteristics. So long as the antioxidant system remains intact, the oxidative degradation of the basestock, and hence the changes in the lubricant's properties, are minimal.
Hydrocarbon oils are also commonly used as transmission fluids and in hydraulic systems. cases, the oil is subjected to pressures, frequent movement and heat. These oils also experience stresses that degrade the basestock, and antioxidants are added to such oils.
In a similar manner, hydrocarbon cooking oils undergo serious thermal stresses. Degradation of the basestock can result in production of acids within the oil which affect the taste of food, and such oils are also protected by an antioxidant system.
Antioxidant species within an oil are gradually depleted with equipment operating time. Eventually, the antioxidants become ineffective, which allows large changes to occur in the physical properties of the oil's basestock. At such point, the oil is no longer capable of adequately carrying out its function, and its useful life ends For lubricants, this can result in excessive component wear and eventual failure of the equipment. For cooking oils, acid concentrations within the oil may increase, affecting the taste of foods fried within the oil.
Since it is undesirable to continue to use an oil beyond the end of its useful life, scheduled oil changes have been devised for various types of equipment. The length of equipment operating time between scheduled changes is selected quite conservatively to ensure that dysfunctional oil is not permitted to remain within the equipment. However, this results in oils with remaining useful life being discarded.
It can therefore be seen that the ability to predict the remaining useful life of an oil would eliminate the need to perform oil changes on the basis of a fixed schedule. This would permit longer use of an oil, thereby providing savings in material and labor costs. Further, abnormal depletion rates for antioxidants within a lubricant or hydraulic fluid sample can detect severe wear problems in operating equipment prior to equipment failures. Abnormal depletion rates in cooking oil can signal improper thermal control within the equipment or other malfunctions.
Various thermal-oxidative and chemical-oxidative stressing techniques having the capability to evaluate remaining useful life are known. However, such techniques are unsuitable for routine use. Thermal-oxidative stressing techniques require the use of high temperatures and pressures and long analysis times in the order of 30 minutes. Chemical oxidative stressing techniques are difficult in operation, require unstable reagents, and also require long analysis times in the order of 120 minutes.
What is needed, therefore, is a method for evaluating the remaining useful life of a hydrocarbon oil which does not require the use of high temperature and pressure or unstable reagents. Such a technique should be rapid, i. e., analysis times of less than a minute, as well as easy to operate and capable of being performed with inexpensive equipment.